As shown in FIG. 13, a bicycle air pump head of the prior art comprises a cylindrical body 51 having a neck 511 for connecting with a tubular member (not shown in the drawing) and a receiving space 512 of cylindrical construction, a pressing member 52 pivoted to the upper portion of the cylindrical body 51 by a pivoting pin 521 and provided with two pressing surfaces 522 and 523. The vertical distance S1 between the pressing surface 522 and the pivoting pin 521 is smaller than the vertical distance S2 between the pressing surface 523 and the pivoting pin 521. The air pump head further comprises a support member 53, a leakproof ring 54, an urging member 55, an elastic body 56, and an end jacket 57. The support member 53 is received in the receiving space 512 and under the pressing member 52 and provided in the lower side thereof with a cut 531. The leakproof ring 54 is fitted over the support member 53 for preventing the air from leaking from the upper portion of the cylindrical body 51. The urging member 55 is disposed under the support member 53 and provided with a through hole 551 in communication with the neck 511 and with a protruded column 552 for use in moving away the closing valve (not shown in the drawing) of the tire air valve. The elastic body 56 is disposed under the urging member 55. The end jacket 57 is fastened to the bottom of the cylindrical body 51 for locking the support member 53, the urging member 55 and the elastic body 56 in the receiving space 512 of the cylindrical body 51. In operation, the center slot 561 of the elastic body 56 is caused to engage a tire air valve before the pressing member 52 is moved upwards, as shown in FIG. 14, with the pressing surface 523 exerting a pressure on the support member 53. As a result, the distance S2 between the pressing surface 523 and the pivoting pin 521 becomes longer, thereby resulting in the support member 53 to cause the urging member 55 to move downwards a distance equal to S2-S1, so as to exert a pressure on the elastic body 56. As the elastic body 56 is caused to deform, the inner diameter of the center slot 561 becomes narrower. The center slot 561 of the elastic body 56 is therefore caused to hold securely the tire air valve to facilitate the air pumping.
As described above, the pressing member 52 must be pressed and turned to cause the elastic body 56 to engage the tire air valve. Such an action can hamper the pumping action. In addition, the end jacket 57 is often provided with a dust-protecting cover. However, the dust can be still caused to enter the pump head through the lower portion of the pressing member 52.